1. Field of the Invention
This invention is related to the general field of gamma-ray imaging. In particular, the invention provides a new method and a new coded-aperture system for detecting gamma radiation emitted from an object under examination and constructing an image corresponding to the spatial distribution of the source of radiation within the object.
2. Description of the Prior Art
Coded-aperture systems were first suggested in the 1960s for use in x-ray astronomy, where the objects of interest are essentially two-dimensional. This idea was exciting at the time because a coded aperture could be far more efficient than conventional pinhole apertures of collimators in collecting radiation such as x-ray photons. It was later understood that more photons were required with a coded aperture than with a pinhole because each photon conveyed less information about the source. Nevertheless, for many kinds of sources, coded apertures offer a considerable net advantage, and they continue to be actively used in x-ray and gamma-ray astronomy today.
The concept of coded apertures was later extended to nuclear medicine, where the objective is a volumetric, or three-dimensional (3D), distribution of a radioactive tracer. This application of coded apertures has been studied extensively during the past 25 years, and its mathematical and engineering aspects are well understood in the art. In particular, it is generally agreed that a single coded image taken with an aperture at a fixed location is an inadequate data set for accurate 3D tomographic reconstruction. The basic reason is that radiation from each point in the object is collected only over a very limited range of angles.
Several approaches have been proposed to overcome this limited-angle problem. One is to use many different apertures, either at the same location or, preferably, at different angular positions around the object to be imaged. As one skilled in the art would readily appreciate, the reconstruction problem then becomes more complicated because the data consist of many different 2D images; nevertheless, it is well within the capability of modern computers and algorithms currently in use in the art. The main drawback is the mechanical complexity of such systems and the fact that the net advantage in collection efficiency is marginal for large three-dimensional objects.
To take full advantage of coded apertures, a detector with high spatial resolution is required, and the scintillation detectors used routinely in nuclear medicine are deficient in this respect. The best scintillation cameras have a spatial resolution of only about 2-3 mm, and there is very little prospect for any substantial improvement.
Accordingly, much effort has been devoted to the development of a new generation of semiconductor gamma-ray detectors for use in nuclear medicine. Such detectors consist of a slab of semiconductor material, such as cadmium zinc telluride, and a multiplexer readout circuit. For example, a 64xc3x9764 detector array with a spatial resolution of 0.38 mm, an order of magnitude better than scintillation cameras, has already been tested successfully in the art (see U.S. Pats. No. 5,245,191 and No. 5,825,033, hereby incorporated by reference) and work on developing new imaging systems to take full advantage of this capability is in progress. The ultimate goal is to build a high-resolution tomographic imaging system based on semiconductor detectors and either conventional pinhole apertures or coded apertures.
The considerable interest found in semiconductor gamma-ray detector arrays arises from the improvements they are able to provide over scintillation cameras in a variety of applications. These devices can be used in nuclear medicine, diagnostic radiology, molecular biology, gamma-ray astronomy, particle-physics and nuclear-weapon applications. Therefore, it is expected that a high-quality semiconductor array combined with improved data processing techniques will have a substantial commercial and scientific impact.
The present application concerns a novel approach in the way data are generated by detector arrays and processed for medical-imaging applications. Because of the background that lead to the invention, this disclosure is based on data and experiments related to the medical field, but its application pertains to all disciplines that can utilize semiconductor sensors for gamma-ray detection.
The goal of medical imaging is to provide a spatial mapping of some parameter, feature, or process within a biological entity. Emission imaging (or nuclear medicine) comprises a class of imaging techniques that produce a functional mapping of the object under observation. Generally speaking, the techniques used in nuclear medicine involve the injection of a radioactive substance into a patient""s body and the measurement of the emitted radiation (gamma rays) by radiation sensitive detectors through a system of apertures in an impermeable medium. Typically, before injection the radioactive tracer (radionuclide) is combined with a substance that is known to be preferentially concentrated in the organ of interest, so that the preferential concentration of the resulting radiopharmaceutical will correspond to an indication of blood flow, metabolism, or receptor density within the organ. Thus, an image of the resulting radioactive distribution within the organ of interest will yield functional information about the organ. Either a single projection image of the emission distribution may be taken (planar imaging) or many projection images may be acquired from different directions and used to compute the three dimensional emission distribution (single photon emission computed tomography, generally referred to as SPECT).
Since photons in the energy range used in nuclear medicine are not substantially refracted or reflected, data are collected by placing attenuating apertures between the patient and the detector plane, so that each detector has an associated field of view defined by the aperture. Photons that are recorded by a particular detector element in the detector plane are known to have originated in a certain small portion of the object space. The number of photons detected by a given detector is proportional to a weighted integral of the activity contained in the region it sees. By utilizing the information collected by many detector elements or cells, each viewing different but overlapping regions of the object space, an estimate of the original activity distribution can be produced by a reconstruction algorithm according to analytical methods and techniques that are well understood by those skilled in the art.
Different kinds of apertures are commonly used to provide the desired select field of view of an object. For example, parallel-hole collimators, focused collimators, single and multiple pinhole attenuators, and several other apertures that can be used to restrict the path of the gamma rays between the radioactive object and the detector in a tomographic imaging system. In each case, each detector element ideally receives radiation from a single line from the object through the aperture of the system. U.S. Pats. No. 5,245,191 and No. 5,825,033 describe the use of a semiconductor detector array in combination with multiple-pinhole apertures to produce an improved tomographic imaging system.
Not all imaging in nuclear medicine is tomographic, however. Most routine studies are performed with simple parallel-bore collimators, and the resulting image is a 2D projection of the 3D object. Common jargon for this approach is xe2x80x9cplanar imaging,xe2x80x9d implying not that the object is planar but that the resulting image is.
The improved semiconductor detectors could be used for planar imaging, but the overall spatial resolution would still be severely limited by the collimator. The bore size in currently available collimators is 1-3 mm, which establishes an absolute lower limit to the resolution in the image. For a thin object in contact with the face of the collimator, the resolution is essentially the same as the bore diameter. With volumetric objects, moreover, the resolution degrades substantially with depth in the object. Photons originating from a point far from the collimator face can pass through several adjacent bores, and in practice the final spatial resolution in planar nuclear-medicine imaging is 10-15 mm. While such images are still very valuable clinically, it is apparent that a better spatial resolution is highly desirable and in some cases necessary.
New fabrication techniques that would produce collimators with finer bores have been considered, but the engineering tradeoffs are not attractive. Using smaller bore diameters with the same bore length, collection efficiency would fall off as the square of the diameter. Choosing instead to shrink all dimensions together, the resolution at the collimator face could be improved, but it would degrade more rapidly with depth in the object. In short, except possibly for specialized applications with thin objects, there is relatively little to be gained by using semiconductor detectors for planar imaging with collimators.
In summary, the parallel-hole collimators routinely used in nuclear medicine suffer from several deficiencies. Their efficiency for photon collection is relatively poor and can be improved only at the expense of spatial resolution, and the resolution degrades rapidly with distance away from the face of the collimator. Even if the poor efficiency could be tolerated, high-resolution collimators would be very difficult to fabricate. Thus, there remains a need for an improved system of data collection and processing for planar imaging of volumetric sources. This invention consists of a new data-gathering approach that improves many of the limitations of existing systems.
The main objective of the invention is the development of a high-resolution alternative to the parallel-hole collimator for planar imaging.
Another goal is a system for obtaining parallel projections of a 3D object using a coded-aperture system instead of a parallel-bore collimator.
Another object of the invention is a system that affords a more efficient collection of photons than a collimator with even coarser resolution is able to provide.
Finally, an objective is the implementation of the invention in a commercially viable system that maximizes the utilization of existing technology.
According to these and other objectives, a coded aperture is placed in proximity of a patient""s body and a 2D coded image is acquired in conventional manner. The basic data-acquisition geometry, illustrated in FIG. 1, is similar to that used in various coded-aperture systems. According to one aspect of the invention, additional coded images are acquired with different spacings between the aperture and the detector. Alternatively, additional coded images could be acquired with multiple movable apertures or by varying the location of the aperture relative to the patient.
Another aspect of the invention resides in the recognition that presently available computer algorithms can process these multiple coded images in such a way as to estimate the integrals of the 3D object over a set of parallel cylindrical tubes extending through the volume of the target object, as illustrated in FIG. 2. Such xe2x80x9ctube integralsxe2x80x9d can be thought of as the output of an ideal collimator where the sensitivity is confined to a tubular region of constant cross-section.
As those skilled in the art readily understand, real collimators do not achieve this ideal condition for many reasons. As noted above, the spatial resolution of a collimator varies with depth, so that the region of integration is more like a thin cone than a cylinder, as illustrated in FIG. 3, and the sensitivity varies with position within the region. In addition, attenuation of the radiation in a patient""s body also causes a variation in sensitivity along the axis of the cone, and penetration of the radiation through the collimator material can further cause significant sensitivity to radiation well outside the cone.
All of these effects can be corrected by data processing according to the method of the invention, thereby producing a resulting image that is superficially similar to one obtained by a collimator, but where the spatial resolution is appreciably improved and remains approximately constant over a substantial range of depths in the object. In addition, many more photons are collected in comparison to a collimator.
Various other purposes and advantages of the invention will become clear from its description in the specification that follows and from the novel features particularly pointed out in the appended claims. Therefore, to the accomplishment of the objectives described above, this invention consists of the features hereinafter illustrated in the drawings, fully described in the detailed description of the preferred embodiment and particularly pointed out in the claims. However, such drawings and description disclose but one of the various ways in which the invention may be practiced.